var myObj = function () {
    this.study = "JavaScript";
}
myObj.prototype.hobby = function () {
    this.hobby = "See girl";
    console.log(this.hobby)//See girl
    return this.hobby
}
myObj.prototype.foots = 4

var newObj = new myObj();
console.log(newObj)//myObj { study: 'JavaScript' }
console.log(newObj.hobby())//See girl